A conventional semiconductor laser device that includes a ridge waveguide, current blocking layers disposed on both sides of the ridge waveguide, and a contact layer, and a method of fabricating this semiconductor device are described below.
FIGS. 11(a)-11(e) are sectional views illustrating process steps in a method of fabricating the conventional semiconductor laser device. Initially, a p type InP first cladding layer 1, an InGaAsP active layer 2, and an n type InP second cladding layer 3 are successively epitaxially grown on a p type InP substrate. Then, an insulating film 4 comprising SiO is deposited on a center portion of the second cladding layer 3 and, using the insulating film as a mask, the first cladding layer 1, the active layer 2, and the second cladding layer 3 are selectively etched to form a ridge waveguide (figure 11(a)). In the step of FIG. 11(b), a p type InP first current blocking layer 12 having a high charge carrier concentration is selectively epitaxially grown on both sides of the ridge waveguide. Further, as shown in FIG. 11(c), an n type InP second current blocking layer 6 is selectively epitaxially grown on the p type InP first current blocking layer 12. In this growth step, the second current blocking layer 6 grows only on a specific crystalline plane of the first current blocking layer 12. Therefore, it is possible to form the n type InP second current blocking layer 6 so that it is not in contact with the side surface of the ridge waveguide. Then, a p type InP third current blocking layer 13 having a high charge carrier concentration is selectively epitaxially grown on the first and second current blocking layers to form a crystalline burying layer comprising the first, second, and third current blocking layers (FIG. 11(d)). In the step of FIG. 11(e), the insulating film 4 is removed. Finally, an n type InP contact layer 8 is epitaxially grown, followed by grinding at the rear surface of the substrate, and formation of an electrode 20a on the ground rear surface of the substrate and an electrode 20b on the n type InP contact layer 8, completing the semiconductor laser device shown in FIG. 12.
In this semiconductor laser device, when a forward bias voltage is applied across the electrodes 20a and 20b, a current flows through the ridge waveguide comprising the n type InP second cladding layer 3, the active layer 2, and the p type InP first cladding layer 1, and holes from the p type InP first cladding layer 1 and electrons from the n type InP second cladding layer 3 are injected into the active layer 2. Radiative recombination of the electrons with the holes produces light in the active layer 2, resulting in laser oscillation.
In the fabrication method described, since the layers of the ridge waveguide and the burying layer are continuously epitaxially grown, no surface of the grown layers is exposed to air during growth. However, the p type InP first current blocking layer 12 is regrown on a side surface of the ridge waveguide that is exposed to air during etching, and the n type InP contact layer 8 is regrown on the upper surfaces of the burying layer and the ridge waveguide that have been exposed to air during the etching of the insulating film 4. The surfaces where regrowth occurs are called regrowth interfaces.
It is known that when the p-n junction between the p type InP first current blocking layer 12 and the n type InP second cladding layer 3 and the p-n junction between the p type InP third current blocking layer 13 and the n type InP contact layer 8 are located at regrowth interfaces, leakage current not passing through the active layer increases. Thus, the forward voltage at which a forward current starts to flow is reduced and the forward current across the p-n junction under continuous operation increases, causing deterioration of laser characteristics, such as a rise in the threshold current and a reduction in light output. In order to avoid this problem, Zn is employed as the dopant impurity in the p type InP first and third current blocking layers 12 and 13. These p type current blocking layers have significantly higher charge carrier concentrations than the n type cladding layer 3 and the n type contact layer 8. The Zn diffuses from the p type current blocking layers 12 and 13 into the n type cladding layer 3 and the n type contact layer 8 during the epitaxial growth process or during heat treatment after the epitaxial growth process, reversing the conductivity type of a thin portion of the n type cladding layer 3 and the n type contact layer 8 in contact with regrowth interfaces. Therefore, the p-n junction between the p type InP third current blocking layer 13 and the n type InP contact layer 8 is not located at the regrowth interface 9a of the upper portion of the burying layer shown in FIG. 12, but at a position 14a in the n type contact layer 8. The p-n junction between the p type InP first current blocking layer 12 and the n type InP second cladding layer 3 is not located at the regrowth interface 9b of the side surface of the n type cladding layer 3, but at a position 14b in the n type cladding layer 3. Consequently, the forward voltage of the p-n junction under continuous operation is not reduced and does not cause deterioration of laser characteristics.
Although deterioration of the laser characteristics under continuous operation can be prevented by diffusing Zn from the p type current blocking layers 12 and 13 into the n type cladding layer 3 and the n type contact layer 8, Zn diffuses not only into the n type cladding layer 3 and the n type contact layer 8, but also into the n type InP second current blocking layer 6. Zn compensates the dopant impurities of the n type second current blocking layer 6, whereby the charge carrier concentration of the n type current blocking layer 6 is reduced and the current blocking effect due to the p-n-p transistor effect is reduced. That compensation causes an increase in leakage current and deterioration of laser characteristics.